The invention relates to a method of installing and cementing a casing in an oil and/or gas well.
During well completion operations it is common practice to install a well casing by first suspending the casing in the well and then pumping a cement slurry into the annular space between the outer surface of the casing and the borehole wall. After the cement has set to a hardened mass, perforations may be extended through the casing and the cement body into the production zones of the earth formation around the well in order to allow inflow of valuable formation fluids such as oil or gas into the well.
The purpose of the cement body around the casing is to fix the casing in the well and to seal the borehole around the casing in order to prevent vertical flow of fluid alongside the casing towards other formation layers or even to the earth surface. Thus, it is essential that a good bonding is created between the cement body and both the casing and the borehole wall.
A problem generally encountered during cementation of the casing in a well is that due to various factors, such as the existence of varying pressure and temperature gradients along the length of the casing and shrinkage of the cement body during hardening thereof, relative displacements occur between the casing and the hardening cement mass which may result in poor bonding between the cement body and the casing. Such poor bonding may result in the presence of a so-called micro-annulus between the casing and the cement body, which micro-annulus may extend along a substantial part of the length of the casing. The occurrence of a micro-annulus is particularly dangerous in gas wells as substantial amounts of gas might escape therethrough to the surface.
Various attempts have been made to improve bonding between well casings and the surrounding cement bodies. It is common practice to use cement compositions with additives that improve adhesion of the cement body to the formation and to the wall of the casing, and to use foam cements that shrink during hardening thereof only to a minor extent. It is also know, for example from U.S. Pat. Nos. 3,918,522 and 4,440,226 to provide the casing with an inflatable packer which is filled with cement. A problem encountered when using such packers is that they are fragile and require a complex cementation procedure.